<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        .dropdown {
            width: 50px;
            border: 1px solid #333;
            text-align: center;
        }

        .dt {
            border-bottom: 1px solid #333;
        }

        .dd {
            display: none;
        }
    </style>
</head>

<body>

    <body>
        <div class="dropdown">
            <div class="dt">微博</div>
            <div class="dd">
                <ul>
                    <li>私信</li>
                    <li>评论</li>
                    <li>@我</li>
                </ul>
            </div>
        </div>
        <script>
            var dt = document.querySelector('.dt');
            var dd = document.querySelector('.dd');
            dt.onmouseover = function () {
                dd.style.display = 'inline';
            }
            dt.onmouseout = function () {
                dd.style.display = 'none';
            }
        </script>
    </body>
</body>

</html>